A press machine for press-forming a work-piece has a structure which has a fixed plate, a slide plate opposite to the fixed plate, a fixed die disposed on the fixed plate and a movable die disposed on the slide plate facing the fixed plate to open and close the movable die against the fixed die by moving the slide plate relatively to the fixed plate. In a small press machine, there is a single drive source provided in a center of a slide plate. Using a large slide plate, the single drive source disposed in a center of the slide plate cannot uniformly press the slide plate. Therefore, using a plurality of drive sources to cause a uniform pressing force on a slide plate, each of the plurality of drive sources presses a respective engaging portion disposed on the slide plate to form a press plane on the slide plate. As the plurality of drive sources, two, four or six ones, for example, have been used.
When a slide plate is descending against a fixed plate to close a movable die against a fixed die and to increase a pressing force, magnitudes of loads working on the movable die through a plate to be formed are changing and working positions of the loads on the movable die are, also, varying. The variations of the magnitudes and the working positions of the loads cause imbalance on load working on the slide plate. A distance from a working position of a load on the slide plate to a drive source, also, is varied. Then, imbalance in load moments acting to the drive sources is caused.
When servo-motors are used for drive sources of a press machine, revolutions of the servo-motors are delayed by loads working to the drive sources. So, since a drive source subjected to a large load is more delayed in proceeding than a drive source subjected to a small load, a slide plate is caused to incline relatively to a fixed plate. The inclination of the slide plate causes a die to incline and often to be injured. When the inclination of the slide plate is small, the die is not injured but may reduce accuracy in press-forming a work-piece.
As a countermeasure, an inclination of a slide plate has been corrected by detecting/measuring the inclination of the slide plate during a progress of the press-forming and adjusting a driving signal supplied to each of the drive sources to reduce/eliminate the inclination of the slide plate. Such a feed-back control can prevent the slide plate from inclining during press-forming.
However, when a slide plate inclination is prevented during press-forming by the feed-back control, a cycle of press forming takes a long time. In a press-forming of a work piece, it is usual that a same kind of work-pieces is repeatedly formed to produce a large number of work-pieces. If a cycle of press-forming takes a long time, there is a problem that a production of a large number of work-pieces takes an extremely long time.